Omnidirectional transponder

ABSTRACT

A transponder for automatic identification and corresponding systems. The transponder may include patch antennas for reflecting transponder data as information sidebands of a microwave signal received from a reading unit, for example. The antennas may by parallel to, but oppositely directed from one another. Additionally the antennas may act against at least one intermediate earth plane, and are linearly polarized so that their polarization direction is rotated through 90° in relation thereto. In this arrangement, the transponder may be read from a short distance and from both sides of the antennas, and communication with the transponder may not be disturbed by the presence of a rearwardly-located reflection plane.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SE97/01931 which has an Internationalfiling date of Nov. 18, 1997 which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a microwave transponder for automaticidentification systems that have a back scatter function.

2. Related Art

A back scatter system can be defined as a system in which the datacarrier, the so-called transponder, includes antennas and modulationcircuits. These components modulate information sidebands with the datato be read from a microwave signal falling on the transponder from areading unit, thus generating a reflected signal containing informationfor reception and decoding by the reading unit, without supplying freshenergy. Certain embodiments also enable data to be written into thetransponder by modulating the amplitude of the signal from the readingunit with detection data, and memory programming in the transponder.This technique is well known and will not therefore be described indetail here.

One problem with microwave-based transponders of this kind, i.e.transponders that operate at 0.9 GHz and even higher frequencies, isthat they are normally unable to communicate omnidirectionally, due tothe directivity of the antenna system embodied in the transponder. Thismakes it necessary to orientate the transponder so that a given side ofthe transponder will face towards the reading unit, which is difficultto achieve when, e.g., the receiving unit is concealed in a package, ahandbag, pocket or like receptacle.

Although the earth plane can be reduced to approximately the same sizeas the antenna plane with the aid of a patch antenna, so as to obtainequal sensitivity in both forward and rearward directions, transponderreadings may become sensitive to reflections from a rearwardly lyingreflection plane, should this plane be located at a distance that causesleakage of the information carrying signal from the transponder antennathrough the signal reflected via the reflection plane. Packaged objectscontained in handbags and the like are often reflective and causeuncertain readings. The same applies to the human body, when thetransponder is carried in a pocket.

The necessity of taking into consideration the orientation of the datacarrier and its proximity to surrounding objects in order to obtainpositive readings is a troublesome limitation in each of these cases.Transponders equipped with dipolar antennas are subjected to similarproblems, since these transponders are also influenced by the possiblepresence of a reflection plane in the proximity of the antenna.

Another drawback with antennas of this type is their low antenna gain,i.e. only a minor part of the incident signal is reflected back to thereading unit, due to lack of directivity, which shortens the range ofthe system.

SUMMARY OF THE INVENTION

The present invention fully solves these problems and provides atransponder that can be read from a short distance and from both sidesof the antenna.

In accordance with one preferred embodiment, there is provided atransponder with which communication will not be disturbed by thepresence of a rearwardly located reflection plane.

The present invention thus relates to a transponder which is adapted foruse in automatic identifying systems and like systems, wherein thetransponder includes a patch antenna which functions to reflecttransponder data in the form of information sidebands to a microwavesignal falling on the patch antenna and sent from a reading unit, andwherein the transponder is characterized in that it includes twomutually parallel but oppositely directed path antennas.

In one preferred embodiment, the transponder obtains an almost fullyisotropic function. In other words, this means that it can be incommunication with in all directions in the absence of the troublesomeneutral settings found, for instance, in the rod directions of dipoles.Corresponding blind directions are, of course, also found with patchantennas that have a small earth plane, since isotropic antennas aretheoretically impossible.

In another particular embodiment of the invention, data can be writteninto the transponder, e.g., by coding amplitude modulation of themicrowave signal, e.g. with a so-called Manchester code, with the datato be written. Such a bit stream in the form of an amplitude modulatedmicrowave signal is captured by detection circuits in the transponder,rectified, amplified and caused to influence the memory register of thetransponder.

The present invention employs two patch antennas that operate fromrespective sides of a common earth plane, or alternatively towardsmutually separate earth planes that are separated solely by means of athin foil. In one aspect of the invention, each of these two antennas isprovided with an individual modulation circuit that is supplied from acommon application-specific integrated circuit (ASIC), in which thetransponder data is stored. This antenna is turned towards the readingunit and reflects signals back to said unit with a directivity, orbeaming effect, that provides a very good range.

This enables the transponder to be read from both directions. It alsoenables the transponder to be produced cheaply, e.g. by laminating theantenna in a known manner. Another advantage is that the ASIC, which isoften the most expensive component of the transponder, is common to bothantennas.

Accordingly, two separate antennas are employed which, in one preferredembodiment, are not mutually coupled by microwave techniques, but are,instead, mutually connected to respective modulation circuits bylow-frequency signal conductors. Because no microwave connection isrequired between the two antennas, the transponder can be manufacturedmuch more simply.

In another particular embodiment, the transponder is communicated withcircular-polarized microwaves, which further enhances the orientationtolerance. One of the antennas may be turned through 90° when theantenna are linearly polarized and in communication withcircular-polarized signals. This provides additional freedom onorientation, since only the E-field and not the H-field has neutralsettings in a transponder with a small earth plane. The E-field andH-field from patch antennas with a small earth plane are disclosed inthe literature, from which the effect referred to is clearly apparent.

The directional characteristic of the transponder is essentially thesame, irrespective of whether it is read or written, and consequentlyremarks concerning reading of the transponder will also apply to writingin the transponder.

With the intention of reducing the size of the transponder, its batterycan be placed within the circle circumscribed by the patch antennas,instead of on one side thereof.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a block schematic illustrating two versions of thetransponder;

FIG. 2 illustrates and an exemplary embodiment of the invention; and

FIG. 3 illustrates another exemplary embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating two versions of thetransponders. The transponder shown to the left of FIG. 1 differs fromknown transponders insomuch that electronics and memory register 1supply two patch antennas 2, 3 that are placed concentrically together,but with the antenna planes directed 180° from each other.

In the transponder to the left of FIG. 1, each of the antennas utilizesits modulation/detection circuits 4, 5, whereas the transponder shown tothe right utilizes a common modulation/detection circuit 6. Bothtransponder types have principly the same function. The left-handtransponder is simpler to implement, but at the cost of an extramodulation element 4 or 5.

The technology of back scatter transponders is well known, with anincident microwave signal on which information sidebands are formed byvirtue of the modulation components 4 and/or 5 periodically changing theantenna impedance in accordance with the data pattern from the memoryregister in block 1, such that the incident signal will be reflected indifferent ways depending on whether a zero or a one is outputted. Themodulation components 4, 5, 6 may comprise a diode, a field effecttransistor or some other non-linear element that can be controlled fromblock 1.

When the transponder is a writable transponder, i.e. when data in theincident microwave signal can be written into the memory register inblock 1, the modulation components 4 and 5 may consist of a diode.Separate components may alternatively be used for detecting andmodulation, although this technology is known and will not be discussedhere.

Because the antennas 2 and 3 are each directed in a particulardirection, the transponder is able to reflect its information sideband,irrespective of whether the microwave signal from a reading unit fallsfrom the right or the left in FIG. 1.

When the antennas have the form of patch antennas with a givendirectivity, i.e. when their earth planes are made larger than theantenna plane, the strength of the reflection will be stronger and thereading and/or writing range of the transponder longer.

Another advantage afforded by an earth plane that is larger than theantenna planes is that the transponder can be placed in the immediateproximity of a reflection plane without disturbing the function to anygreat extent. If this was not so, there is a danger that transpondercommunication will be uncertain when the transponder is placed, e.g., onor in the vicinity of a reflecting object in a bag or the like, due tothe fact that the reflecting signal will be totally or partiallyextinguished by the signal reflected via the reflection plane.

Patch antennas that have small earth planes, i.e. with a lateralextension that lies between the own extension of the antenna element andits double-extension, have a directivity and back lobe that graduallytransforms from a lobe breadth of about 90°, and a very small back lobeto a lobe width of 360°, i.e. the back lobe is equally as large as thefront lobe.

Of course, zero settings are found in a blind axis in all antennas, evenif the earth plane is small. Otherwise, the antennas would be isotropic,which is a physical impossibility. These zero settings, however, onlyoccur in an axis along the E-field of the antenna plane, whilesensitivity is retained in all directions in the H-field of the antennaplane when the earth plane is small.

FIG. 2 illustrates an exemplary embodiment in accordance with theinvention. Consequently, the inventive embodiment illustrated in FIG. 2,in which the two antenna planes are rotated through 90° in relation toone another, provides a transponder that has an almost isotropicfunction. When reading is effected with a unit that iscircular-polarized, so that it is able to irradiate and receivetransponder information sidebands irrespective of the direction in whichits linear-polarized antenna is orientated, a highly effectiveomnidirectional transponder-reading has been achieved.

Because the earth plane is slightly greater than the antenna plane, theinventive transponder is relatively insensitive to being placed on areflection plane, meaning that the back lobe from respective elementsand its reflection in a rearwardly lying reflection plane will bemoderate and in practice unharmful to the direct reflected wave.

Thus, in this embodiment of the invention, the size of the earth planeis adapted so that the transponder will not be sensitive to placement onreflection planes, whilst obtaining an almost isotropic function at thesame time.

FIG. 2 thus shows the two antenna planes 10, 11 rotated through 90° inrelation to one another in order to eliminate blind directions inaccordance with the aforegoing. Each of the antennas acts against anearth plane 12, which may be common to both antennas or, for practicalreasons, separated by a foil which may be conductive or insulating. Thisfoil may be provided with an adhesive to enable the two covers 13, 14 ofthe transponder to be joined together.

There is nothing to prevent the earth planes 12 from being separateplanes, provided that they are not spaced too far apart, so as todisturb the function of their corresponding respective antennas.

In another embodiment of the invention, not shown, antenna planes andearth planes are constructed in accordance with conventional multilayertechniques, with which the antenna planes are pressed onto both sides ofan intermediate earth plane, and where a transit hole is provided forpassing signal conductors and earth conductors between respectiveantenna sides.

In the case of the described example, the incident microwave signal ispassed, via conductors 15, 16, to the modulation components, e.g. thediodes 17, 18, whose impedance varies in accordance with a pattern fromthe memory register in the circuit 19.

In the illustrated case, one of the modulation components is suppliedvia a low-frequency signal conductor 20 that connects one side of theantenna to the other side thereof. In another embodiment, not shown,this signal conductor may carry microwaves from one side to the other,so as to enable a common modulation component to be used.

When the transponder is powered by a battery 21, e.g. when it is notpowered by microwaves or low-frequency electromagnetic fields, it can bepositioned concentrically with the antennas and inwardly of theradiating edges of the antenna elements. The lateral extension of thetransponder will therewith be smaller, without disturbing or impairingits function.

FIG. 3 illustrates an embodiment in which a battery 22 is positioned onone side of the antenna elements, so as to obtain a construction similarto a credit card. The solution and function in other respects are thesame as that described with reference to FIG. 2. Transponder data ispassed between the antennas via the intermediate connection 23, whichmay be a wire conductor as shown, or may be a penetrating-plating whenthe antennas have the form of multilayer cards.

Although the invention has been describe with reference to variousembodiments thereof, it will be understood by the person skilled in thisart that the structural design of the transponders may be varied.

These illustrated embodiments do not therefore limit the scope of thepresent invention, since they can be modified within the scope of thefollowing claims.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A transponder comprising: at least two patchantennas for reflecting transponder data as information sidebands of amicrowave signal received from a reading unit, wherein the two antennasare parallel to, but oppositely directed from one another, wherein saidat least two antennas act against at least one intermediate earth plane,and wherein said at least two patch antennas are linearly polarized sothat their polarization direction is rotated through 90° in relationthereto.
 2. A transponder according to claim 1, wherein each of the twopatch antennas has a respective modulation and detection circuitcontrolled from a common application-specific integrated circuit througha low-frequency signal conductor.
 3. A transponder according to claim 2,wherein each path antenna acts against a common earth plane fromopposite directions.
 4. A transponder according to claim 2, wherein eachof the patch antennas acts against its respective earth plane, saidearth planes being separated from one another by a foil so as to besubstantially adjacent to one another.
 5. A transponder according toclaim 2, wherein the transponder is read with a circular-polarizedmicrowave signal from the reading unit.
 6. A transponder according toclaim 2, wherein the transponder comprises a single unit that includeselectronics and includes a memory register to which modulation unitshaving modulation and detection circuits are connected, and wherein eachof said patch antennas is connected to a corresponding modulation unit.7. A transponder according to claim 2, wherein the transponder includesa single unit that includes electronics and a memory register; wherein asingle modulation unit containing modulation and detection circuits isconnected to said unit; and wherein both patch antennas are connected tosaid modulation unit.
 8. A transponder according to claim 1, whereineach patch antenna acts against a common earth plane from oppositedirections.
 9. A transponder according to claim 8, wherein thetransponder is read with a circular-polarized microwave signal from thereading unit.
 10. A transponder according to claim 8, wherein thetransponder comprises a single unit that includes electronics andincludes a memory register to which modulation units having modulationand detection circuits are connected, and wherein each of said patchantennas is connected to a corresponding modulation unit.
 11. Atransponder according to claim 8, wherein the transponder includes asingle unit that includes electronics and a memory register; wherein asingle modulation unit containing modulation and detection circuits isconnected to said unit; and wherein both patch antennas are connected tosaid modulation unit.
 12. A transponder according to claim 1, whereineach patch antenna acts against its respective earth plane, said earthplanes being separated from one another by a foil so as to besubstantially adjacent to one another.
 13. A transponder according toclaim 12, wherein the transponder is read with a circular-polarizedmicrowave signal from the reading unit.
 14. A transponder according toclaim 12, wherein the transponder comprises a single unit that includeselectronics and includes a memory register to which modulation unitshaving modulation and detection circuits are connected, and wherein eachof said patch antennas is connected to a corresponding modulation unit.15. A transponder according to claim 12, wherein the transponderincludes a single unit that includes electronics and a memory register;wherein a single modulation unit containing modulation and detectioncircuits is connected to said unit; and wherein both patch antennas areconnected to said modulation unit.
 16. A transponder according to claim1, wherein the transponder is read with a circular-polarized microwavesignal from the reading unit.
 17. A transponder according to claim 16,wherein the transponder comprises a single unit that includeselectronics and includes a memory register to which modulation unitshaving modulation and detection circuits are connected, and wherein eachof said patch antennas is connected to a corresponding modulation unit.18. A transponder according to claim 16, wherein the transponderincludes a single unit that includes electronics and a memory register;wherein a single modulation unit containing modulation and detectioncircuits is connected to said unit; and wherein both patch antennas areconnected to said modulation unit.
 19. A transponder according to claim1, wherein the transponder comprises a single unit that includeselectronics, and includes a memory register to which modulation unitshaving modulation and detection circuits are connected, and wherein eachof said at least two patch antennas are connected to a correspondingmodulation unit.
 20. A transponder according to claim 19, wherein thetransponder includes a single unit that includes electronics and amemory register; wherein a single modulation unit containing modulationand detection circuits is connected to said unit; and wherein both patchantennas are connected to said modulation unit.
 21. A transponderaccording to claim 1, wherein the transponder comprises a single unitthat includes electronics and a memory register; wherein one modulationunit having modulation and detection circuits is connected to said unit;and wherein each path antenna is connected to said single modulationunit.